Abstract: By use of direct trajectory optimization method,the selection of overall design parameters of an anti ballistic missile for a 1000km range ballistic target is discussed. The following factors are taken into account in our research: the variation of parameters of the solid rocket motor,the constrains to the collision angle required by the seeker side window,the constrains to the maximum Mach number and the terminal velocity for the consideration of ablation problem,the selection of initial flight path angle and time of launch and other considerations such as the maneuver load limit and gravity compensation. It is found that a very large interceptable region can be obtained by properly selecting the design parameters and by use of the hybrid rocket motor.